The present invention relates to a continuous drier for plywood sheets in which the drier comprises an elongated housing divided into a plurality of consecutive sections in each of which a heater is arranged as well as a ventilator or blower for blowing air over the heater and installations for guiding the hot air thus produced in transverse direction over the plywood sheets which are transported by a conveyor with variable speed through the drier, and which include a device for automatically controlling the speed of the conveyor.
In modern installations, the drier is arranged directly downstream of a machine which cuts the plywood sheet from a wood block, and the drier therefore must receive the plywood sheets in the sequence in which they are produced from the cutting machine. Since the cutting sequence of the cutting machine has to be continuously adapted to the quality of the wood to be cut, and since the width of the individual plywood sheets varies greatly in accordance with the thickness of the wood block to be cut, the covering of the conveyor with plywood sheets is irregular. In order to avoid overlapping of the plywood sheets on the conveyor, the latter is practically driven with the maximum speed determined by the maximum number of cutting strokes of the cutting machine and the maximum width of the cut plywood sheets. The density at which the conveyor is covered by the plywood sheets will vary in practice continuously between zero and about 70%, and the average of covering of the conveyor by plywood sheets is usually about 50-60%. While the possibility exists to manually regulate the speed of the conveyor, this is usually not done since the operating personnel are usually fully occupied with other tasks. Furthermore, by manually regulating the speed of the conveyor it is at best possible to carry out rough corrections only.
In correspondence with the varying covering of the conveyor by plywood sheets, the amount of water to be evaporated from the latter varies also considerably. This in connection with the different initial humidities of the individual plywood sheets therefore requires a continuous adaptation of the drying requirements.
German Offenlegungsschrift No. 27 21 965 discloses a method which takes these requirements into account. According to this method, the local heat consumption and the humidity of the air are measured along the conveyor through the drier and the optimal drying conditions are calculated by means of a calculator and automatically adjusted. The adjustment may for instance be carried out by varying the speed of the conveyor transporting the plywood sheets through the elongated drier. The varying density at which the conveyor is covered by the plywood wood sheets is thereby accepted, and while the change of the speed of the conveyor indirectly influences also the density at which the conveyor is covered by the plywood sheets, a dense covering of the conveyor by plywood sheets is thereby neither aimed at nor accomplished.
The varying density at which the conveyor is covered by the plywood sheets has, however, a series of disadvantages. Since it is necessary to operate the conveyor continuously at high speed, the wear on the conveyor is considerable. If considerable distances exist between successive plywood sheets on the conveyor, the edges of the plywood sheets are excessively dried, which is detrimental to the quality thereof. Due to the high speed of the conveyor, the time at which the plywood sheets remain in the drier is relatively short. To compensate for this high drying temperatures are necessary, which entail a reduction of the quality of the plywood sheets, or drying air streams of great volume are required, which entails a high energy consumption for the drive motors of the ventilators.
German Offenlegungsschrift No. 15 28 302 discloses a conveyor for transporting plywood sheets through a drier in which the speed of the conveyor is, in accordance with the width of successive plywood sheets and the operating speed of the cutting machine, controlled in such a manner that the plywood sheets follow each other on the conveyor in close succession. Since in this case only the speed of the conveyor is controlled, the necessary conditions for properly drying the plywood sheets are not met. Evidently, a change of the speed at which the plywood sheets are transported to the drier will also change the time during which the plywood sheets remain in the drier, and therewith the drying conditions.